no. 2
On the uniqueness of the Cauchy problem for partial differential operators with multiple characteristics
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 7 (1980) no. 2, pp. 257-285. 
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     title = {On the uniqueness of the {Cauchy} problem for partial differential operators with multiple characteristics},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
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     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 7},
     number = {2},
     year = {1980},
     mrnumber = {581144},
     zbl = {0478.35006},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1980_4_7_2_257_0/}
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Zeman, Marvin. On the uniqueness of the Cauchy problem for partial differential operators with multiple characteristics. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 7 (1980) no. 2, pp. 257-285. http://www.numdam.org/item/ASNSP_1980_4_7_2_257_0/

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